1. Field of the Invention
This invention relates to the fabrication of composite rigid foam structures, and to the structures so fabricated. In particular, the structures are composed of a rigid foam material that has at least one face sealed or skinned with a thin skin that is tightly and uniformly adhered to but does not significantly penetrate the foam.
2. Description of the Prior Art
A cellular solid is made up of an interconnected network of solid struts or plates which form the edges and faces of cells, examples of which are shown in FIG. 1. See also, for example, L. J. Gibson and M. F. Ashby, Cellular Solids, Pergamon Press, First Edition 1988. The simplest is a two-dimensional array of polygons, which pack to fill a plane area and are typically called honeycombs. See FIG. 2(a). More commonly, the cells are polyhedra, which pack in three dimensions to fill space. Such three-dimensional cellular materials are called foams. If the solid of which the foam is made is contained in the cell edges only (so that the cells connect through open faces), the foam is said to be open-celled or reticulated. See FIG. 1 and FIG. 2(b). The cell edges or boundaries of the cell of open-cell or open-pore foams are often called ligaments. If the faces are solid too, so that each cell is sealed off from its neighbors, it is said to be closed-celled. See FIG. 2(c). Foams can contain, for example, both open- and closed-cells in the same body. Foams are differentiated by their material or materials or fabrication, whether open or closed, the number or cells per linear dimension and other parameters. A typical designation of cell size is cells or pores per linear inch (ppi).
Foams can be made by a number of processes and from a variety of materials. See, for example, A. J. Sherman et al, “Refractory Ceramic Foams: A Novel, New High-Temperature Structure”, Ceramic Bulletin, Vol. 70, No. 6, 1991, which is hereby incorporated herein by reference. One process for fabricating open-pore foams involves coating/infiltrating an open-pored reticulated vitreous carbon foam by chemical vapor deposition/infiltration (CVD/CVI). This method allows the formation of foams of many different materials, including metals and ceramics. In this case the vitreous carbon foam is used as a skeleton for the CVD material and the skeleton is often, but not necessarily retained. FIG. 1 shows a reticulated rigid tantalum foam made in this way.
Open-pored foam materials are used for a wide variety of purpose, including, but not limited to, filtration devices, heat exchangers, catalyst supports, structural panels, or the like. Both open- and closed-pore foams are used as cores or elements for lightweight structures such as, for example, space mirrors, insulation, thermal protection systems and the like. See, for example, H. A. Scott, “Integral Structure and Thermal Protection System”, U.S. Pat. No. 5,154,373, Oct. 13, 1992. For example, strong, lightweight structural members can be made up of two stiff, strong skins separated by a porous foam core. The separation of the skins by the core increases the moment of inertia of the panel with little increase in weight, producing an efficient structure for resisting bending and buckling loads. Used as heat exchangers, the open pore cellular materials may also require skins to contain the heat exchange media and/or to contain the pressure within the porous core.
The attachment of a skin or skins to foam materials had previously often been problematic because the surface of the foam material is composed of a multitude of discreet points or small areas rather than a continuous surface, which would lend itself to bonding or brazing or other similar attachment means. In addition, the difficulty of attachment is a function of the pores per inch (ppi) and the physical size of the ligaments or other attachment areas. As shown, for example, in FIG. 1, the area which must be bridged by the skin between adjacent ligaments is often at least five times or more the thickness of the ligaments. This is especially problematic when the skins are subjected to structural loads or hermetic seals are required, since any unattached areas weaken the structure or are points of potential leaks. In addition, it is difficult to detect areas that are not attached since they are hidden by the skins. Also repair of unbonded areas usually requires removal of the skin and reattachment. Even more problematic have been the cases where the surface of the porous material is not of simple geometry such as a flat surface or a cylinder. In these cases it had previously been nearly impossible to get total surface attachment of the skin to the porous material. In addition, when the skin and the porous material are made of different materials or when no compatible bonding agent is available, the optimum structure could not be formed.
In order to overcome these difficulties, in situ formation of the skins by chemical vapor deposition (CVD) has been attempted with little success. The results were that either too much of the “skinning material” penetrated into the porous material, increasing its weight beyond the useable range; or repeated cycles of deposition and machining were required at great cost. This was especially true of open-cell foams. Surface control and uniformity, especially with non-simple shapes, were also a problem. Repeatability was substantially non-existent.
These and other difficulties of the prior art have been overcome according to the present invention which provides for the fabrication of firmly and uniformly attached in situ skins on foam materials, utilizing nearly any combination of materials or geometry, and the structures so fabricated.